Boats having planing hulls are designed to operate most efficiently at speeds where the hull planes on top of the surface of the water, and a minimum area of the hull is wetted. Typically, about one-half to two-thirds of the hull area is wetted at planing speeds; however, the wetted area may vary. Depending upon the design, as much as one-half of the hull area may be wetted at lower speeds, or as little as one-fourth of the hull wetted during very high speed cruising.
The type and configuration of a water craft's propulsion system greatly affects the planing characteristics of its hull. This is because the thrust force vector of the propulsion system may include force components which act about the center of gravity of the boat to affect the planing attitude of the hull. Ideally, the most efficient propulsion and hull arrangement will have propeller thrust and hull resistance forces acting substantially parallel to one another, and a minimum wetted hull area.
The original inboard marine propulsion systems having an essentially straight, direct drive between the inboard engine and the screw propeller are typically configured so that force components created by the propeller thrust adversely affect the planing characteristics of the boat hull. This is because the angular disposition of the propeller drive shaft with respect to the boat hull produces a upward as well as forward force component on the hull. The effect of the upward force component is to raise the stern of the hull and rotate the hull about the center of gravity of the boat. As the stern of the hull is raised with an increase in propulsion power and speed of the craft, rotation of the hull about the center of gravity of the boat creates a "bow down" attitude, thus creating a bow wake and appreciably reducing speed and efficiency of the craft due to increased wetted area. Thus, direct drive inboard propulsion systems have not proved to be the most effecient system for use with planing hulls. On the other hand, planing hulls having outboard engine propulsion systems are not similarly adversely affected since the center of thrust of the system is disposed parallel to the planing axis of the craft, and the center of gravity of the boat is as far astern as possible with the outboard engine mounted on the transom of the hull.
Because inboard propulsion systems permit the use of relatively larger power plants than outboard arrangements, and provide the security and accessibility of an onboard installation, various attempts have been made to improve the planing characteristics of boats having inboard drive. One approach has been to use a so-called "V-Drive" in which the inboard engine is mounted as far astern in the craft as possible. The drive shaft of the engine extends forwardly thereof, and is operatively connected with the downwardly and rearwardly extending propeller shaft by a gearbox. This arrangement improves upon the undesired "bow down" effect created by direct drive inboards by moving the center of gravity of the craft farther astern and reducing the downward angle of the propeller shaft. This reduces the bow down attitude, and places the center of gravity over a wider portion of the hull, thus improving lift of the hull and reducing the overall wetted hull area. However, this arrangement is more costly and somewhat less power-efficient than a comparable direct drive inboard since the gearbox "V-drive" connecting the engine drive shaft and propeller shaft reduces the ultimate power delivered to the propeller shaft.
In order to combine the desirable characteristics of inboard drives and outboard drives, so-called inboard-outboard, or Z-Drives have been developed and are in very widespread use today. The engine of this system is mounted inboard of the hull near its stern, and is connected by a suitable drive train with a propeller drive housing mounted on the transom and extending rearwardly thereof. This system desirably places the center of gravity of the craft as far astern as possible, while at the same time permitting positioning of the drive housing so that the center of propeller thrust is parallel with the planing axis of the hull.
A significant development for inboard-outboard drives has been the introduction of arrangements for altering the angular disposition of the propeller drive housing during operation of the craft, thus permitting trimming of the hull to improve its planing characteristics. This is typically provided by an arrangement whereby the lower portion of the propeller drive housing may be moved fore and aft by rotation about a pivot point above the waterline. In this way, the thrust force vector of the propeller may be selectively directed so that a force component is created which urges the stern of the craft downwardly. As the craft rotates about its center of gravity due to this downward force component on its stern, the bow of the craft is lifted, thus improving efficiency of the hull and increasing top speed. A further advantage of the inboard-outboard drive system is that the center of thrust of the propeller is disposed parallel to the planing axis of the hull, and below the center line of hull resistance, thereby creating a rotating couple which tends to raise the bow of the hull as power and speed increase.
One of the most recent developments in propulsion systems for planing hulls is the so-called surface-piercing drive, or surface drive. This system is similar to inboard-outboard drives in that the engine of the system is disposed inboard of the hull, while the propeller drive housing is mounted on the transom of the hull and extends rearwardly thereof. The drive housing is mounted on the transom by a ball-and-socket type joint with the propeller drive shaft connected with engine output shaft by a universal joint, thus permitting the housing to be pivoted both vertically and laterally. Hydraulic actuators extend between the drive housing and the transom of the hull for selective positioning of the housing, in this way effecting both steering and trimming functions.
In significant distinction from typical inboard-outboard drives, the new surface drives are designed so that the screw propeller of the system is only partially submerged during optimum operating conditions of the craft. In this way, drag created by the drive housing and propeller itself are greatly reduced as compared with inboard-outboard drives, resulting in greater overall efficiency and speed. The propeller slippage factor for this type of drive is comparable to or slightly higher than that of a drive having a fully submerged propeller.
As discussed, the trimmable nature of recent Z-Drive propulsion systems permit adjusting of the drive housing for the most efficient planing of the hull on the surface of the water by creating a downward force component on the stern of the hull. However, the nature of surface drive systems does not permit the creation of a similar downward force component. This is because movement of the surface drive housing to create a downward force component on the stern as with a Z-Drive (i.e. pivoting of the Z-Drive housing rearwardly) results in the propeller of the surface drive being moved out of the water. Obviously, this results in an immediate loss of speed and efficiency, since these types of drive systems are designed for maximum efficiency when the screw propeller is about half-submerged.
Accordingly, a trimming arrangement suited for use with a marine drive system incorporating a partially submerged screw propeller (surface drive) which permits creation of a downward force component on the stern of the planing hull of the craft for improving its planing characteristics is desirable and further enhances the performance of surface marine drive systems.